1. Field of the Invention
The present invention relates to an automatic pneumatic pressure control apparatus for automatically controlling an output pneumatic pressure produced by a pneumatic pressure device and a method of controlling such an automatic pneumatic pressure control apparatus, and more particularly to an automatic pneumatic pressure control apparatus for use as an electropneumatic transducer as a pneumatic pressure device or a pneumatic pressure actuator employing such an electropneumatic transducer, and a method of controlling such an automatic pneumatic pressure control apparatus.
2. Description of the Related Art
Electropneumatic transducers constitute one category of automatic pneumatic pressure control apparatus which are actuated by an electric input signal supplied as a manipulated variable (also referred to as a "manipulated variable signal") according to an automatic control process. In response to the electric input signal, the electropneumatic transducer generates a pneumatic pressure as an output signal known as a controlled variable or a controlled variable signal, and an electric signal corresponding to the pneumatic pressure is fed back to the electropneumatic transducer.
A flow control valve or the like is connected to an outlet port of the electropneumatic transducer, making up a pneumatic pressure actuator as an automatic pneumatic pressure control apparatus. In response to an electric input signal (manipulated variable) applied to the electropneumatic transducer, the pneumatic pressure actuator produces an output signal (controlled variable) representing the opening of the valve member of the flow control valve, and the output signal is fed back to the pneumatic pressure actuator.
One known automatic pneumatic pressure control apparatus is disclosed in Japanese laid-open patent publication No. 7-4401 entitled "Electropneumatic transducer apparatus and actuator system employing same", filed by the present applicant.
Generally, automatic pneumatic pressure control apparatus employ the principles of the PID (Proportional plus Integral plus Derivative) control mode. According to the PID control mode, an offset-free, stable, and accurate response can be achieved when supplied with a signal that changes stepwise from a value "0" to a value "1", such as a step signal (step input), for example, as a reference signal (also called a "reference input" or an "input signal").
Pneumatic devices suffer a dead time which is consumed after a manipulated variable is supplied until a controlled variable is actually operated. If such a dead time is large, a so-called overshoot or a large oscillation tends to occur due to the integral control during the dead time.
In an effort to solve the above problem, there has been developed an automatic pneumatic pressure control apparatus for automatically controlling a pneumatic device which suffers a dead time in response to a step input according to the PID control mode. Specifically, at the start of the control process, the automatic pneumatic pressure control apparatus starts to control the pneumatic device according to PD (Proportional plus Derivative) control. If a detected error, which is the difference between a reference signal and a feedback signal, becomes smaller than a predetermined value, i.e., after the elapse of the dead time, the PD control mode is changed to the PID control mode to eliminate an overshoot or an oscillation.